The present invention relates to a method for producing a multilayer paper web in a twin-wire former of the kind in which the wires define a curved forming zone, to which at least two different stocks are delivered from a multilayer headbox, and from which forming zone white water is ejected, collected and substantially returned to the papermaking process.
The invention also relates to an apparatus for producing a multilayer paper web for realization of the method, including a twin-wire former of the kind in which the wires define a curved forming zone, a multilayer headbox for delivering at least two different stocks to the curved forming zone, and means for collecting and returning to the papermaking process at least a substantial portion of the white water ejected from the curved forming zone during the forming of the multilayer paper web.
The expression "forming zone" is used herein in the meaning generally accepted in the art, i.e. a zone extending from the point where dewatering of the stock through at least one wire begins up to the point where dewatering has progressed so far that the fibers forming the paper web can no longer float around in the suspension liquid, but bear against each other substantially immovably.
Water that is separated from a fiber suspension or fiber mat in a paper machine is called white water. White water usually contains fiber residues, sometimes also filler, dyes, rosin-size and the like, and is generally returned to the papermaking process. The flow circuit that is arranged for the return of white water to the process, and which comprises piping, storage containers, cleaning means and control equipment for the return flow, is usually called a white water system. A white water system is said to be open if a major portion of the total white water flow leaves the system and is said to be closed if only a small portion of the total white water flow leaves the system. A flow circuit for so-called short white water around a former is called a short circulation, and the term "short white water" refers to white water that is returned to the stage of the process from which it originated. Analogously, the term long circulation refers to flow circuit for so-called long white water, i.e. white water that is returned to a process stage other than the one from which it was separated.
The white water that is returned in a short circulation is used to dilute the stock from a higher consistency to headbox consistency, e.g. in newsprint making from above 2.5 percent by weight to below 1 percent by weight, and this return takes place without cleaning of the white water. At newsprint mills with Fourdrinier machines it is known that white water separated at the end of the Fourdrinier section has a considerably lower content of suspended matter than that of white water separated at the beginning of the Fourdrinier section. Compare for example, GB No. 1,352,672 and CA No. 1,021,506. This cleaner white water is returned as a rule in a long circulation to the mill grinder room, but part of it can be conducted to a final cleaning before discharge into a suitable receiving body of water. It is also known to divide up the white water from a Fourdrinier machine into three fractions with a purity increasing towards the end of the Fourdrinier section, with the cleanest fraction being conducted for final cleaning. The technical aspects of system design and closure of systems for newsprint machines of Fourdrinier type are given in a report "Skogsindustrins miljovardsprojekt" ("Forest Industry Environmental Project") from SSVL, Stiftelsen Skogsindustriernas vatten- och luftvards-forskning (The Water and Air Pollution Research Foundation of the Swedish Forest Industries), pages 151-155 and 178-190.
Further, a roll type twin-wire machine is disclosed in U.S. Pat. No. 3,846,232 in which the forming zone curves along the shell face of a forming roll of suction roll type. The forming zone is followed by a slightly curved dewatering zone with a maximum length of about three times the diameter of the forming roll. In order to obtain the slight curve, the inner wire is supported along the dewatering zone by several guiding means, such as rotatable rolls or one or more fixed and narrow foils or deflectors. It is stated that white water separated at the forming roll is collected in a first saveall and a second saveall, which are located on either side of the wires, and the white water is conducted to the intake of the mixing pump for the headbox to be used as dilution water. It is also stated that white water which is separated from the formed paper web downstream of the forming zone and which usually has a lower content of fibers than white water from the forming roll, is collected in separate savealls to be conducted to a point in the water system where a lower fiber content is desired.
Due to the forced drainage of the stock at a forming roll as compared with a Fourdrinier section, the content of suspended matter in the excess white water will be higher at the forming roll, and compared with white water from the end of the Fourdrinier section, white water from the forming roll can have a content of suspended matter that is more than 50 percent higher. When making multi-ply paper in a twin-wire former of roll type the white water can therefore be comparatively rich in fiber and contain both long and short fibers. In cases where it is desired to make a multi-ply paper with at least one layer of long fibers and at least one layer of short fibers and to dilute long-fiber thick stock and short-fiber thick stock with white water to form stocks of predetermined headbox consistencies, and wherein the white water used for dilution is rich in both short and long fibers, no appreciable layer purity can be attained. The layer of short fibers will contain a large proportion of long fibers and the layer of long fibers a large proportion of short fibers. This unfavorable result can be made still worse if the headbox is not of the kind that keeps the stocks separated not only through the headbox but also for a distance downstream of its slice openings for the stocks. At worst, the layer purity will have been reduced so much that when determining the proportion of long fibers and the proportion of short fibers in the layers by counting the fibers, no conclusive difference can be established between the different layers.